Aligned multiwalled carbon nanotubes (MWNT) are most commonly grown using chemical vapor deposition (CVD). This method, however, has a serious drawback: aligned MWNTs can only be grown only on certain materials because of the high temperature at which the growth takes place (about 700° C.). As a result, current technology does not allow patterned aligned MWNT to be grown on any type of substrate. For example, it is not possible to grow nanotubes by an acetylene-based CVD process on metals that have low melting temperatures (e.g., Al, Pb, etc.) or metals that react with acetylene (e.g., Cu, etc.).
Another known method of growing aligned MWNT is the electrophoretic method, which is a cold cathode fabrication process further described in Y. Nakayam, S. Akita, Synth. Met. 117 (2001) 207-210. MWNTs produced in this manner have limited application because of the weak adhesion of the carbon powder to the substrate. Other processes may include the arc discharge method or microwave plasma deposition method.
Ink-jet or screen printing of patterned cold emitters is another fabrication method, which involves the printing of specially formulated inks having conducting or semiconducting particles in an insulating matrix. The application of this method is hindered because the printable field emitters produced by this method have high threshold voltages (˜20 V μm−1) and require special conditioning procedures to obtain uniform electron emission at sufficiently low fields.
Field electron emission (FE) from MWNT has become a widely known phenomenon since the discovery of nanotubes in 1991. Field emitters comprising aligned arrays of MWNTs have higher current densities and a better stability over long duration of time as compared to conventional field emitters. Moreover, there are no methods and processes capable to transfer matter in high vacuum while retaining the pattern and preferential orientation of the matter without using complicated multi-step lithographic processes involving masks, stamps and/or pre-patterned substrate preparations.
Different techniques have been proposed for improvement of mechanical properties of carbon nanotube emitters, such as embedding the bottom part of vertically aligned carbon nanotubes into a SiO2 film or burying nanotubes in a polymer matrix. However, these techniques only improve mechanical contact, but they do not decrease electrical and thermal resistance of the contacts, which are the critical parameters for applications based on vertical geometry such as cold field emitter, nanoelectrode arrays for chemical and biosensors and vertical nanotransistors.
There is, therefore, a need for a method and apparatus for transferring an array of oriented carbon nanotubes having a certain pattern, and improved thermal and electrical characteristics.